The invention relates to a process and a device for high-speed flame spraying of refractory wire or powder weld filler for the coating of surfaces, in which an all-gas high-speed flame spray burner is used to coat surfaces with any refractory wire or powder spray weld filler.
In that regard two or more gas mixing systems that operate independently from each other, which can operate with different fuel gas-oxygen mixtures, are integrated in the device.
A multiplicity of processes, devices and technologies are known from prior state of art that no longer meet the high requirements of modern technology.
DE-PS 81 18 99 proposes an arrangement for atomizing metallic and non-metallic substances, which can be viewed as the basic principle for high-speed spraying using fuel gas and oxygen. The arrangement primarily involves a system comprised of a combustion chamber and expansion nozzle, with which wire, powder or molten spray weld fillers can be sprayed using primarily hydrogen as detonating gas. Therefore, when using the proposed device, only one heating or fuel gas, primarily hydrogen, can be used each time, which according to the compressed gas principle is introduced into the combustion chamber. According to DE-PS 81 18 99, the ignition of hydrogen is done manually when emerging from the expansion nozzle, electrically by short circuit or by means of an electric arc.
Hydrogen can be ignited by means of the molten heated spray weld filler, which is combined with the detonating gas through the combustion chamber via an access.
The design concept proposed in DE-PS 81 18 99 in many respects does not meet the requirements imposed today on high-speed flame spray equipment.
On the one hand the fuel gas, which according to DE-PS 81 18 99 is hydrogen, is introduced according to the compressed gas principle into the combustion chamber, which no longer fulfills either the legal type approval requirements for autogenous burners or accident prevention provision UW-VGB 15.
In addition, hydrogen without additional oxidation gas, such as oxygen, does not produce sufficient heat to be able to spray refractory spray weld fillers such as molybdenum, wolfram and oxides. On the other hand, hydrogen burns reducing and for that reason is unsuitable for spraying metal oxides, since the hydrogen flame takes oxygen from the spray weld filler in the molten or plastic state.
Another high-speed flame spray system is known from EP-O 049 915. This high-speed flame spray system has a water cooled expansion nozzle that is supposed to be suitable for spraying wire or powder weld fillers. In contrast to the concept of DE-PS 81 18 99, the heating gas used may be optionally nitrogen, propane or MAPP gas, in addition to oxygen. The individual fuel gases are introduced according to the compressed gas principle into a large mixing chamber and mixed with oxygen. The fuel gas-oxygen mixture moves through bores into the water cooled expansion jets, where it is combined in the combustion chamber with the powder or wire weld filler.
This method in accordance with EP-A1-0 049 915 also suffers from numerous technical applications and safety problems.
The design concept proposed in EP-A1-0 049 915 precludes the use of acetylene as heating gas, since, because of the high rate of flame propagation of acetylene, the risk of flame backfire or backflash, due to the compressed gas principle, is extremely high.
Excluding the use of acetylene in connection with oxygen significantly limits applications, since due to the high flame energy particular weld fillers such as refractory metals and oxides can be sprayed and melted only with an acetylene-oxygen flame at 3160.degree. C. The extremely high rate of flame propagation of an acetylene-oxygen mixture, approximately 11.5 m/sec., compared with propane-oxygen, which in a mixture ratio of 1:5 is about 3.6 m/sec., which results in practice in significantly higher rates of flame propagation and therefore higher kinetic particle speeds, cannot be taken advantage of in the proposed system. Gas mixing systems of the type described above do not fulfill the accident prevention requirements of VGB 15 nor the type approval requirements for autogenous devices.
The acetylene-oxygen flame possesses dominant properties that cannot be obtained by any other fuel gas-oxygen mixtures. For that reason it is ideal for thermal spraying of refractory weld fillers.
However, the use of acetylene as a heating gas to operate high-speed flame spray systems in connection with oxygen is problematic due to the specific structure of the acetylene molecule.
Acetylene is a chemical combination of carbon and hydrogen. It is so-called unsaturated hydrocarbon, whose molecule possesses an inner tension and seeks equilibrium. Therefore, acetylene is not a stable substance, but instead is inclined to decompose into its components, i.e. carbon and hydrogen. For example, when acetylene is heated to a temperature of about 300.degree. C., and if it is also under pressure, then any decomposition initiated will be continued by the entire gas quantity. The energy released in the form of heat is sufficient to bring neighboring acetylene particles to decomposition temperature. That process occurs so rapidly that when decomposition is initiated compressed acetylene decomposes in an explosive manner. That condition can occur, for example, if acetylene is introduced into a combustion chamber of a high-speed burner and is ignited; the expansion produces a combustion chamber pressure in the order of magnitude between 2 and 3.5 bar, so that because of the backdraft on the fuel gas line the aforementioned acetylene decomposition occurs.
Because of the condition described above, back-ignition in the gas mixture area occurs where fuel gases, in this case acetylene and oxygen, are combined. The aforementioned negative occurrence prevents the fuel gas, acetylene and oxygen from combusting in the combustion chamber and in that way a high-speed flame can be produced.
Moreover, it is known that in the current state of the art oxide-free spray coatings, such as those made of hastelloy, tribaloy or extremely pure nickel can only be produced using plasma vacuum chamber spraying. That technology is very complex and extremely cost intensive.